In my related co-pending application, Syringe Coupling System, Ser. No. 953,609, filed Oct. 23, 1978, I describe a needle hub with improved frictional retention to an outer surface of a syringe adapter. Such hub is adapted to be twistingly removed from the syringe by a wrenching structure on a needle protector.
The present invention relates specifically to an improved coupling between a "plug type" needle hub that forms a liquid-tight seal on an inner surface of a syringe adapter. The hub is intended to be permanently secured to such adapter by an external skirt that squeezes the tubular adapter between the skirt and plug of the hub. Such structure is very useful for low capacity (lcc) syringes used to inject insulin. The plug prevents loss of any significant amount of insulin within the tubular adapter.
A previous plug type hub structure for a low capacity insulin syringe is described in U.S. Pat. No. 4,040,421. Here the external gripping skirt of the hub includes an annular lateral circumferential rib 54 that grips the syringe adapter. However, this rib 54 has disadvantages in that (1) it prevents sterilizing gases from entering a molding tolerance pocket at 16 which might become contaminated during a manufacturing process, and (2) it is difficult to mold because a lateral rib tends to distort when longitudinally stripped from its mold. Also, the hub is of a relatively soft polypropylene material for ease of assembly and moldability. A polypropylene skirt of the hub has limited dimensional stability and can slightly expand due to cold flow during long term storage, thereby loosening its grip.
A transverse rib was believed necessary between the hub skirt and syringe adapter to get any significant frictional retaining force. U.S. Pat. No. 3,043,304 describes spirally cutting threads in longitudinal ribs to get a thread interlock for holding power. A longitudinally extending rib would be expected to freely pull off at the syringe adapter because there were no lateral shoulders in either the hub skirt or the syringe adapter. Such longitudinal ribs have been used in "pull off" needle protectors, as described in U.S. Pat. No. 3,121,747.
The applicant unexpectedly found that longitudinal ribs do provide a very firm retaining lock on a plug-type hub where the hub and syringe adapter have substantially different hardnesses.